You need to know two things about your beer in order to do this properly. First, what volume of CO2 do you want in the beer? You can determine this from the style. My Dubbel will want something in the range of 2.3 to 2.9 Volumes. Next, what is the volume of beer you're carbonating? In my case, this is a 2.5 gallon batch.

On the Simplicity Candi Syrup web site, they offer an instruction document for carbonating beer with Candi Syrup. The information below is derived from that document.

CO2 (Volumes)

Grams
of Candi Syrup per Gallon

2

22.47

2.1

24.66

2.2

26.84

2.3

29.03

2.4

31.21

2.5

33.4

2.6

35.58

2.7

37.77

2.8

39.95

2.9

42.14

3

44.32

I want to carbonate my 2.5 gallon batch of Dubbel to approximately 2.5-2.6 volumes of CO2, In the chart above, that means I'll need approximately 34 grams of Candi Syrup per gallon. With 2.5 gallons to carbonate, that's 85 grams of syrup by weight. With that calculation, I was ready to prime my beer.

I placed a small pan with a lid on my scale and set the tare to zero.

Poured candi syrup in slowly until I reached 85 grams.

Reset the tare back to zero.

Slowly poured in water until I hit 34 grams (actually, I overshot accidentally to 51 but decided this would be OK given the next step).

Brought the mixture to a boil, which didn't take long since it was a very thin layer in the pan.

Put the pan into an ice bath to cool it.

Transferred the beer from the fermenter to the bottling bucket.

Checked the temperature of the syrup mixture, then poured it into the bottling bucket.

Stirred the contents of the bucket well to ensure a proper incorporation of syrup.

Bottled and capped the beer as usual.

Two weeks later, when I opened the first bottle, the Dubbel poured into the glass with a great color and a nice thick head on it. The carbonation level was comparable to a bottle of Chimay Red that I had right after it, so I think the experiment was a success! The Candi Syrup, being added near the end like this, also improved the flavor of the beer. It had been a little bland when I tasted a sample before bottling it and adding the syrup. The finished beer ended up with dark fruit elements that are common for the style.